1. Field of the Invention
The present invention relates to a headbox for a machine for the production of a fiber material web from a fiber suspension, for example in the manufacture of paper and cardboard and, more particularly, to a headbox for such a machine having a vertically adjustable partition.
2. Description of the Related Art
A paper making machine, as is known, has a specific machine width corresponding to the desired width of the fiber material to be produced. The fiber material web is continuously formed from the fiber suspension in that a machine-wide fiber suspension flow is applied on a machine-wide, continuous and revolving wire belt. A headbox serves to form a fiber suspension flow which is generally uniform across the machine width so that the finished fiber material web will possess generally uniform properties across its width. The headbox has in customary fashion a machine-wide main chamber which is defined by an essentially horizontal bottom and two side walls. The side walls extend parallel to the longitudinal direction of the machine with the spacing between the sidewalls approximately the same as the machine width, i.e., the desired web width. The fiber suspension flows in the longitudinal machine direction first through a machine wide feed channel, then through the main chamber and finally a machine wide outlet channel adjacent the main chamber, the height of which tapers in nozzle fashion up to a machine-wide outlet gap. The headbox must be designed such that the machine-wide fiber suspension flow leaving it has a flow velocity which is generally uniform across the machine width, and so that the fiber material is uniformly distributed in the fiber suspension. To provide optimal uniform distribution of the fiber material, fiber suspension flow should be such that a so-called microturbulence is present which, in turn, should be distributed uniformly across the machine width. The microturbulence prevents the fibers from balling up into flakes, from which a non-uniform fiber distribution would result in the finished fiber material web. The microturbulence is caused by the fiber suspension flowing in known fashion into the feed channel through a bundle of turbulence tubes.
At the transition point from the feed channel to the main chamber there is a rotatable perforated roll provided which in known fashion extends transverse to the longitudinal machine direction through the main chamber and is rotatably mounted in the sidewalls. A second perforated roll of the same type is provided at the transition point from the main chamber to the outlet channel. A similar arrangement of perforated rolls is disclosed in U.S. Pat. No. 2,881,674. The level of the fiber suspension flow passing through the main chamber is always lower than the clearance of the main chamber, i.e., an air cushion is contained in the main chamber above the suspension level. The pressure of the air cushion can be varied in known fashion, in accordance with the desired flow velocity at the outlet gap.
Also known is arranging in the center area of the main chamber, between the two perforated rolls, a partition which extends generally vertically and cross-wise to the longitudinal machine direction. The partition is vertically adjustable, so that between the bottom of the headbox and the bottom edge of the partition there exists a machine-wide channel section which is variable in height. This channel section forms a partial local obstruction to the fiber suspension flow between the two perforated rolls, which obstruction serves as well to maintain the microturbulence. The height of the channel section is adjusted to obtain a desired flow rate.
A headbox of known construction is illustrated in FIG. 1A and includes a downstream outside surface of the partition which extends in a generally vertical direction from the downstream bottom edge of the partition. Moreover, in a region slightly downstream from the bottom edge there is a slat arranged on the bottom. The distance between the adjustable height bottom edge of the partition and the slat determines the flow cross section for the fiber suspension flow. With this prior design, it was occasionally observed that when the flow rate is relatively low and, therefore, the clearance of the channel section is made relatively small as well, there occur downstream from the partition relatively large eddies which are not uniformly distributed across the machine width. The second perforated roll on the downstream side of the partition is apparently unable to sufficiently dissolve these eddies so that, as a result, the finished fiber material web has a non-uniform basis weight across the web width. This formation causes relatively large flakes or "clouds" to be recognized in the finished paper, indicating that the fiber material is not sufficiently and uniformly distributed in the web.
In said headbox of known construction, the lower part of the partition is tapered in such a way that the clearance of the channel section (defined by the bottom and by the partition) becomes smaller in the direction of flow. In this prior design, therefore, the end of the channel section has the form of a slot-type discharge gap. Such a slot-type discharge gap causes the above-described problem of non-uniform distribution of the fiber material in the fiber suspension.
What is needed in the art is a headbox which produces minimal eddies in the fiber suspension flow, and a flow velocity and microturbulent state of flow which are generally uniform across the machine width.